What is the action of Biuret reagent on proteins?

Hint: The biuret reagent is used to identify the presence of proteins. Biuret reagent is the alkaline solution of copper in the state $C{u^{2 + }}$ in hydroxide ions $O{H^ - }$. Due to the presence of copper in the state $C{u^{2 + }}$ it is blue in colour.

Complete step by step solution:
First of all we will read about transition elements.
Transition elements: Those elements which are in groups from three to eleven. They are called a transition because they are in between the s- block elements and p-block elements. For example: Scandium, iron, zinc, etc. They have fully or at least one electron in their d-orbits. For example: scandium has one d-electron, zinc has ten d-electrons. Due to the different number of electrons present in their d-shells they show different valencies. For example: Scandium having one electron can show three valencies as one, two and three. Because the atomic number of scandium is $21$and its electronic configuration is $[Ar]3{d^1}4{s^2}$. By losing only one s-electron it will attain one valency and by losing both the s-electrons it will attain valency two. And by losing two s-electrons and one d-electron it will achieve three valencies which are stable states of scandium. Because after losing three electrons it will attain electronic configuration of noble gas argon.
Due to the different number of d-electrons in the d- orbitals they can form complex compounds.
Proteins: Proteins are made up of amino acids. Amino acids are made of amine group and carboxylic acid group attached to the alkyl. The bond used to bind the amino acids in proteins is peptide bond.
Biuret reagent: The biuret reagent is used to identify the presence of proteins. Biuret reagent is the alkaline solution of copper in the state $C{u^{2 + }}$ in hydroxide ions $O{H^ - }$. Due to the presence of copper in the state $C{u^{2 + }}$ it is blue in colour.
Biuret is blue in colour due to the presence of copper in the state $C{u^{2 + }}$. When proteins are added in the biuret reagent then,it breaks the peptides bonds of proteins and forms a complex compound which is purple in colour. Hence it is used in the detection of proteins.

Note: Transition elements form coordination compounds because they have empty valence shell orbitals i.e. d-orbitals which can accept pairs of electrons from ligands (which donate electrons to the central metal atom also known as Lewis base).